1. Field of the Invention
The invention relates to a device for optical heterodyne or homodyne detection of an optical signal beam, which device comprises a local oscillator, an optical system comprising a polarization-sensitive beam-splitting layer for splitting the signal beam into two orthogonally polarized sub-beams and a beam-combining layer for combining signal beam radiation with local oscillator radiation, said device further comprising a detection system for converting the combined radiation into at least one electric signal which is suitable for further processing.
2. Description of the Related Art
Devices for optical heterodyne detection are sued in optical signal transmission. By mixing the signal beam in a heterodyne detection device with an optical beam from a local oscillator, a considerably better result with regard to the signal-to-noise ratio and the discrimination of background radiation is obtained as compared with direct detection of the signal beam. The invention also relates to a mirror system for splitting a first and a second incident radiation beam into four exit sub-beams each, each sub-beam from the second incident radiation beam exiting in the same direction as one of the sub-beams from the first incident radiation beam, said mirror system comprising four partially transparent mirror portions which enclose four angles. Such a system is suitable for use in a device according to the invention.
The principle of heterodyne detection of optical radiation has been extensively described in the article "Optical Heterodyne Detection" by O. E. DeLange in the journal "IEEE Spectrum" of October 1968, p. 77-85. As has been stated in this article, it is important that the states of polarization of the signal beam and the local oscillator beam correspond as much as possible. A possible solution to achieve this is to split the signal beam into two sub-beams having a fixed and mutually orthogonal state of polarization. The two sub-beams are then combined with local oscillator radiation which is polarized in the same state.
In principle, four components are required for splitting and combining the beams: two polarization-sensitive beam splitters for splitting the signal beam and the local oscillator beam, respectively, and two beam-combining elements for combining the sub-beams formed. By firstly combining the signal beam with the oscillator beam and only thereafter splitting it by means of a polarization-sensitive beam splitter into two orthogonally polarized sub-beams, one of the beam-combining elements can be dispensed with. Since a beam-combining element not only has two inputs but necessarily also two outputs, two polarization-sensitive beam splitters remain required to receive and detect all signal radiation.
A device as described in the opening paragraph is known from EP-A 0,345,889 which corresponds to U.S. Pat. No. 5,003,625, see particularly FIG. 3 of said document. In this device the beam-splitting and beam-combining elements are integrated in an optical component comprising two beam-splitting layers. One of the layers is a polarization-sensitive beam-splitting layer, while the other beam-splitting layer is neutral with respect to the state of polarization of the incident light and is used as a combining element.
The two layers must be disposed with regard to each other and with respect to the entrance beams in such a way that the four exit sub-beams of the signal beam and the four exit sub-beams of the local oscillator beam exit pairwise in the same direction. This imposes strict requirements on the angles at which the beam-splitting faces should extend to each other. This is particularly the case when the beam-splitting layers intersect each other and the radiation beams are incident on four different locations on the layers as is shown in the embodiment in FIG. 3 of EP-A 0,345,889.